Both membrane-proximal and truncation mutations in CSF3R have recently been reported to drive the onset of chronic neutrophilic leukemia (CNL). Here we show that although truncation mutation alone cannot induce leukemia, both proximal and compound mutations (proximal and truncation mutations on same allele) are leukemogenic with a disease latency of 90 and 23 days, respectively. Comparative whole-genome expression profiling and biochemical experiments revealed that induced expression of Mapk adaptor protein Ksr1 and enhanced Mapk signaling are crucial to leukemogenesis by CSF3R proximal and compound mutants. Moreover, inhibition of Mek1/2 by trametinib alone is sufficient to suppress leukemia induced by both CSF3R proximal and ruxolitinib-resistant compound mutations. Together, these findings elucidate a Mapk-dependent mechanism of CSF3R-induced pathogenesis, and they establish the rationale for clinical evaluation of MEK1/2 inhibition in CNL.